Epoxy resin has an excellent adhesive properties to various types of base materials, and a cured material obtained by curing epoxy resin with a curing agent is relatively superior in heat resistance, chemical resistance, electric properties and mechanical characteristics or the like. Therefore, it has been widely used for coating materials, adhesive agents and various molding materials.
Conventionally, the mainstream of epoxy resin composition has been a two-component type wherein the addition of a curing agent or a curing accelerator is required just before use. The two-component type epoxy resin composition has characteristics in that it can cure a resin composition at ordinary temperatures or low temperature. However, on the other hand, the two-component type epoxy resin composition is required to measure each component and blend the measured composition each other just before use. In addition, since the available time of the blended composition is short, it was difficult to apply the blended composition to an automatic machine. These were the problems to be solved in the epoxy resins in that the use conditions of the epoxy resins were limited. In order to solve such problems of the epoxy resins, one-component curable epoxy resin composition was desired.
In order to obtain such one-component curable resin composition, a curing agent having properties which does not initiate a reaction at room temperature but initiates the reaction by heat to cure the composition, a so-called latent curing agent is required. As examples of latent curing agent, for example, dicyandiamide, dibasic acid dihydrazide, boron trifluoride amine complex salt, guanamines, melamines and imidazoles are proposed.
However, for example, the compositions obtained by blending dicyandiamide, melamine or guanamines with epoxy resin were excellent in storage stability but had a problem that a heating for a long time at high temperature of 150° C. or more were required in order to cure the compositions.
Furthermore, it was also widely carried out to combine the curing accelerators with these latent curing agents in order to shorten a curing time, however, a problem that the storage stability is remarkably decreased occurs.
On the other hand, although dibasic acid dihydrazide and imidazoles can initiate the curing reaction at relatively lower temperature, the storage stabilities of these systems were poor. Although boron trifluoride amine complex salt has advantages that the storage stability is excellent and the curing time is short, it has disadvantages that corrosive properties against metals is high and water resistance is poor.
A lot of microcapsule type latent curing agents for epoxy resin, which are obtained by treating a surface of the amine curing agents such as amine epoxy adducts by polyisocyanate, have been proposed. However, they have not satisfied sufficiently yet performances such as storage stability and curing properties when heating.
Some proposals have been made as follows: for example, the microcapsule type curing agents wherein a curing agent is a core component and a thermoplastic resin is a shell component (Patent document 1); the curing agents for epoxy resin obtained by reacting a fine powder, which is comprised of a mixture of a compound having a primary amino group or a secondary amino group and a compound having a tertiary amino group, but having neither primary amino group nor secondary amino group, with an isocyanate compound (Patent document 2); the powder curing agents for epoxy resin having a spherical shape, which are the solid adducts synthesized from amine compounds and epoxy compounds (Patent document 3). However, any of them has not satisfied the storage stability and curing properties when heated.